1. Field of the Invention
The field of this invention is heat recovery from wet wood waste. More particularly, the fuel of interest is hog fuel generated by sawmills and similar wood processing facilities.
2. Description of the Prior Art
In the past few years it has become increasingly evident to processing operators utilizing wood as a raw material, especially in sawmills integrated with pulp or composite products operation, that profitability requires the use of as much of the incoming raw wood material as possible. Wood waste, unsuitable for composite products, is now commonly burned for its heating value. Since these wastes, as burned, are typically low in heating value, many burning systems require supplemental oil or gas fuel in order to facilitate smooth operation. Recent escalation in the cost of oil or other clean energy resources has focused attention upon the efficiency of these heat recovery processes. This is particularly true since many of the wood burning systems were designed in the 1960s, primarily as an alternative to land-fill disposal, before the value of energy dramatically increased.
Wood waste from sawmilling and related raw wood handling operations has two characteristics that make efficient recovery of the heating values difficult. First of all, typical sawmill waste, commonly called in aggregation "hog fuel," has a very irregular particle size range. The wastes are generated from every wood handling and processing operation. These wastes range from sander dust of 100 mm diameter average particle size to bark and log yard debris that may exceed dimensions of 1' in diameter by 4' in length.
The second difficult characteristic is that hog fuel, as fired, may be very wet, even exceeding the 68% moisture limit of self-sustaining combustion on occasions. Each sawmill source of waste has its own characteristic moisture content. However, since the major source of waste is bark which is often removed hydraulically, the fuel flow to the boiler is generally wet. Most wastes are also stored out in the weather so that the normally drier sawdusts and shavings soak up rain water during wet periods of the year.
A common practice in the past has been to burn wet hog fuels using supplemental oil to sustain combustion and achieve reasonable steaming production rates. This approach, of course, is less attractive in view of the high cost of oil.
The more recent approach has been to reduce moisture content of the hog fuel prior to burning for heat recovery into a combination oil-wood waste boiler. Studies show that reducing the moisture content of the fuel burned improves steam production and reduces boiler stack emissions. The dry fuel requires less excess air and thus boiler heat losses are reduced, improving overall thermal efficiency. The resulting higher combustion zone temperature apparently ensures incineration of particulate material before it floats out the stack.
R. C. Johnson in "Some Aspects of Wood Waste Preparation for Use as a Fuel," TAPPI Vol. 58, No. 7 (July 1975) describes almost doubling steam production rates for a wood waste recovery system by reducing fuel moisture content from 63% to 28% by weight. The Johnson article is incorporated by reference herein as showing the state-of-the-art.
Johnson describes a process for predrying hog fuel, utilizing a rotary dryer. The wet wood waste is screened and any oversize reduced to 100% minus 4" in diameter. The wood waste is then introduced into a rotary dryer where combustion gases dry the waste to about 30% moisture. After drying, a fines portion is separated from the dried fuel. The fines are then fed to a burner system which provides heat for the rotary dryer. The balance of the dry fuel, that is the coarse fraction, is fed onto the grate of a conventional wood waste boiler for heat recovery through steam production.
While improving the overall system's thermal efficiency and heat release rates, the burning of fines to dry the hog fuel is not without difficulties. The fines burners must be relatively precisely designed and operated to avoid high maintenance down time. Fuel for the dryer fines furnace, of course, must be predried and sized, which relatively stringent requirements can lead to difficulties or require supplemental high-cost fuels, particularly upon start-up.
Fluidized bed reactors for burning combustibles that vary widely in particle size and moisture content are described in the literature. Reese, in U.S. Pat. No. 3,882,798, assigned to Combustion Power Company, Inc., shows a system for burning municipal refuse by injecting the light fraction of the waste into a fluidized bed of inert or chemically reactive fine granular solids. Sowards, in U.S. Pat. No. 4,060,041 teaches burning wet wood waste in a fluidized bed of olivine.